As the population becomes aware of the potential adverse effects to the body and to the environment associated with use of ingredients derived from fossil fuels, the personal care industry has advanced its search for “natural” ingredients. Although the term “natural” currently has no industry standard definition, efforts are under way by industry trade organizations to devise a more uniform, concise meaning. Currently, it is generally recognized that materials derived from renewable and/or sustainable or otherwise non-fossil fuel sources, are considered to be natural. Personal care compositions containing these materials may be marketed accordingly. Currently, it is the industry consensus that petrochemicals and petrochemically-derived materials are not “natural”. Compounds made by certain chemical processes may also be considered unsuitable for use in “natural” personal care compositions.
In skin care, the tactile impression or skin feel of a product, such as the feeling upon initial application (initial feel), the feeling during spreading of the product over the skin (rub-out), and the feeling after application is complete (after feel), contribute to the product's commercial success or failure. Other characteristics, such as fragrance and taste or lack thereof, may be equally important. Generally, emollients are the components of a composition that are directly related to the “feel” or tactile impression properties, because they provide lubrication, humectancy, and occlusion.
The spreading rate and viscosity are properties of an emollient that contribute to skin feel or tactile impression. Rapidly spreading/low viscosity products are perceived as “light”, whereas slow spreading/higher viscosity products are perceived as “heavy.” While heavy feeling products are sometimes preferred, for example, in skin care products such as massage oils, ointments, and barrier creams, lightness as well as a lack of oiliness and/or greasiness is preferred in many applications. Other terms used to describe lightness of a personal care composition may be “dry,” “velvety,” and “silky.” Additionally, in most instances, it is preferable that the emollient(s) is substantially and/or completely free of color and/or taste. It is also desired that they be non-flammable and toxicologically benign.
Esters have been used widely in personal care and pharmaceutical applications for many years.
Natural ester fats, oils and waxes (vegetable oil emollients) are used in their native state providing emolliency, lubricity, structure, and solvency to personal care applications. Vegetable oil emollients are natural; thus, a personal care formulator preparing a “natural” product may choose a vegetable oil over a naturally derived or petrochemically-derived synthetic, even when the magnitude of benefits realized is lesser as compared to that which would be obtained by use of the petrochemically-derived alternative. Common vegetable oils used in personal care compositions include coconut oil, corn oil, cottonseed oil, canola oil, olive oil, palm oil, peanut oil, safflower oil, sesame oil, soybean oil, sunflower oil and jojoba oil. The viscosities of exemplary vegetable oils are listed in Table 1. As can be seen, the viscosities of these oils are relatively high, and therefore provide a “heavier” skin feel.
TABLE 1Viscosity of Common Vegetable OilsCommon NameViscosity, Centistokes at 25° C.Corn Oil65Canola Oil67Olive Oil87Soybean Oil69Jojoba Oil130
Synthetic esters are compounds that are typically formed from the esterification of fatty acids and alcohols. Many fatty acid and alcohol starting materials are available to the ester chemist, and a tremendous range of variation of properties may be obtained. Exemplary properties are viscosity, melting point, surface tension, reactive index, specific gravity, and viscosity/temperature behavior. Also, by using proper esterification and other purification techniques, pleasing aesthetic properties such as skin feel, and the absence of odor and color may be obtained.
Low viscosity emollients that can deliver a light feeling to the skin can be made using entirely non-petrochemically-derived starting materials; however, the availability of suitable starting materials is limited. For example, monoesters of low viscosity and light skinfeel can be synthesized by esterifying ethanol from fermentation of corn, sugar cane, beets, and/or other plants with fractionated vegetable fatty acids of lower chain length, normally derived from coconut or palm kernel oil. Although this method can provide low viscosity, the monoesters derived therefrom have a low molecular weight and tend to be volatile, and therefore odorous, which makes their use limited.
Naturally derived light synthetic esters for use in personal care compositions may also be derived from the esterification of glycerol from vegetable sources with fractionated vegetable derived fatty acids of lower chain length. The most common of these is glyceryl tricaprylate/caprate (Lexol GT 8/65, Inolex Chemical Company, Philadelphia, Pa., USA.) in which the capric and caprylic acid is obtained from splitting and fractionation of coconut or palm kernel oil. The glyceryl tricaprate/tricaprylate material has a viscosity of approximately 25-30 centistokes (at 25° C.) and a spreading value of 2.9 (5 minutes, cm2) and has been characterized as having a light skin feel.
Many starting materials derived from petrochemical origins may be used to produce synthetic esters. However, the principle remains that odorless, lower viscosity synthetic esters having higher spreading values are considered preferable when one wishes to devise a formulation providing a light skin feel. For this reason, often petrochemically-derived materials fitting this definition are selected for use over any glyceryl tricaprylate/caprate material or other natural materials, since these natural materials do not provide the same performance benefits. Examples of such petrochemically-derived synthetic esters widely used within the personal care industry are neopentyl glycol diheptanoate (LexFeel 7, Inolex Chemical Company, Philadelphia, Pa., USA) and isononyl isononanoate (Dermol 99, Alzo International, Sayreville, N.J., USA.) U.S. Pat. Nos. 4,322,545, 4,323,693, and 4,323,694 to Scala, and U.S. Pat. No. 6,365,629 to Zofchak et. al, incorporated herein by reference, also describe wholly or partially petrochemically-derived esters useful in personal care formulations for providing “dry emolliency” and/or “light feeling” to the formulation.
Although there are many chemical pathways to form esters, the most common chemical reaction is typically the direct condensation of a carboxylic acid and an alcohol to yield an ester and water.
This reaction is reversible and will go to about twenty to eighty percent conversion until equilibrium is reached, depending upon the starting materials used. To achieve higher levels of conversion, the water of reaction is removed so that only the forward reaction prevails. Hydrolysis is the reverse reaction wherein the ester reacts with water to form the parent acid and alcohol. Hydrolysis of esters may be catalyzed by acid or alkali. Since pH is a measure of acid or alkali concentration, the kinetic rate of hydrolysis increases as the system pH deviates from neutrality, downward or upward.
In many personal care applications, the final form of the product contains water. Depending on the solubility of water in the phase containing the ester, the pH, and the fundamental strength of the ester linkage, the rate of hydrolysis will be affected.
The fundamental strength of the ester linkage is mainly affected by stearic factors. Alkyl substituents in close proximity to the ester linkage hinder attack on the linkage by water, and thus the rate of hydrolysis is slowed. For example, it has been reported that the alkaline hydrolysis rate constant (kOH, M-1 min-1) for ethyl benzoate and isopropyl benzoate are 15.5 and 3.2 respectively at 60° C. (Larsen and Johansen, 1985.) The isopropyl ester rate under these conditions was about five times lower than that of the ethyl ester illustrating the effect of stearic crowding.
When an ester hydrolyzes, the parent alcohol(s) and acid(s) is(are) released. The molecular weight of the products of cleavage (alcohol and/or acid) are lower than the molecular weight of the ester. For this reason, in many cases the vapor pressure of the parent alcohol and/or acid is higher than that of the ester, and the compounds are volatile enough to create stimulate olfactory nerves, and an odor that can be disagreeable is observed. For this and other reasons, more hydrolytically stable esters may be preferred.
Recent demand for palm oil has expanded due to its increased use in fuel, food, toiletry and other personal care applications. The primary palm plantations are located in Southeast Asia, and particularly in Malaysia and Indonesia. This strong increase in demand has led to deforestation, as many forests have been destroyed to make room for the growing of palm. This has led to increased carbon emissions due to the practice of burning to rapidly induce deforestation. It has also led to microclimate changes in the region as dense forests that retain water are replaced with open palm plantations. Also, deforestation has reduced the available habitat for species such as the orangutan. For this reason, products that are not derived from palm (“palm-free” or “not palm-sourced”) may be preferred.
Accordingly, there is a need in the art for esters that can provide a light skin feel when incorporated as an emollient into personal care formulations. Furthermore, there is a need in the art for esters that can provide a light skin feel when incorporated as an emollient into personal care formulations and which meet the current “natural” standard. Ideally, such materials would have viscosities and spreading values similar to the petrochemical alternatives. Furthermore, the esters would preferably be hydrolytically stable, low odor, have good odor stability, and not be derived from palm vegetable sources.